It is well known to obtain low TCR (Temperature Coefficient of Resistance) resistors. Said resistors will change very little in their resistance when subject to uniform temperature changes. For example, wirewound or thin film or foil resistors may change as little as 3 ppm/° C. In other words, if the ambient temperature changes from 25° C. to 125° C. (a 100° C. temperature difference) the resistor will change (3 ppm/° C.) (100° C.)=300 ppm ΔR/R. The resistor property of low TCR is therefore useful and desirable where high precision is required and ambient temperature changes may occur.
However, if the same resistor is subject to electric power (current) without a change in ambient temperature the resistance can also change several hundred ppm's depending on the power applied. This phenomena is sometimes described as the Joule effect or resistor self-heating. Both resistance changes due to changes in ambient temperature and resistor changes due to electric power phenomena are additive.
For applications where resistors are used as current sensors (i.e. 4 contact devices) such changes in resistance due to self-heating would, in many cases, be so significant so as to make such resistors unsuitable for accurate current sensing. To resolve this problem, one uses several resistors connected in parallel to distribute the heat due to power across the plurality of resistors so that the temperature of each resistor is reduced and the effect of self-heating is reduced. There are significant disadvantages to this approach, however, as the resulting component is larger (several resistors as opposed to a single resistor), more costly in materials, requires labor for assembly, and the component takes up more space on a printed circuit board than a single resistor. Thus, problems remain.
Therefore, it is a primary object of the present invention to improve upon the state of the art.
It is a further object of the present invention to provide a resistor with suitable properties for use as a high precision power resistor.
A still further object of the present invention is to provide a resistor suitable for use in current sensing applications.
Another object of the present invention is to provide a resistor that demonstrates only small changes in resistance due to power.
Yet another object of the present invention is to provide an improved resistor designed to take into account properties of the resistive foil adhesive cement and substrate to provide a cumulative effect of reduction of resistance change due to power.
A further object of the present invention is to provide a resistor that can be manufactured on a large scale and at a reasonable cost.
One or more of these and/or other objects, features, or advantages of the present invention will become apparent from the Specification and claims that follow.